6.2. SR 11-19-2007 - Laserfiche WebLink

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TABLE 1 SUMMARY OF MODELED LOADINGS GENEUATED IN ER04 TP Load (Ibs/yr) `TSS Load (Ibs/ r) Runoff VoWme <: (ac-fU r) Baseline Condition 1,230 440,600 1,050 Present Condition 1,920 678,600 1,740 2020 Condition w/o 60% TP removal 2,230 772,700 2,000 2020 Condition w/60% TP removal 890 54,100 2,000 As shown in Table 1, there is a significant improvement in the 2020 loading rates far TP and T55 to less than the Baseline condition with the 60% TP removal requirement in place. This implies same retrofit of water quality treatment for areas already developed. Conservatively, it was assumed that there would be no effect on runoff volumes based on this water quality treatment requirement. An infiltration requirement is necessary to mitigate the increase in runoff volumes. It should be noted that the 60% TP removal requirement was applied as a general standard to all urban land uses without specific guidance or analysis on how it would he achieved. Typically standard National Urban Runoff Program (NURP) designed ponds provide adequate water quality treatment to achieve the 60% TP removal. Ultimately the City will be required to apply a water quality treatment requirement that meets nondegradation goals. Structural and non-structural BMPs developed as a part of BMP summary sheet Sa-1 should be designed to meet the water quality treatment requirement. To address revisions to the city's water quality requirements, the City of Elk River proposes aBMP Se-1 modification to the citywide water quality treatment standard and adoption of a water quality treatment ordinance as modification to its current SWPPP. OF AN INFILTRATION REQUIREMENT As discussed above, application of a water quality treatment requirement was not assumed to provide a reduction in runoff volume. Therefore an additional infiltration requirement is needed to mitigate the increase in runoff volumes for urban land uses. Based on the Soil Survey, the City is dominated by hydrologic soil group A and B soils and therefore a good candidate for infiltration best management practices. At present, the Minnesota Pollution Control Agency National Pollution Discharge Elimination System (MPCA NPDES) General Construction Permit sets an infiltration guideline such that the runoff generated by the 0.5-inch rainfall over the impervious surfaces of a development is infiltrated/filtrated. Continuous simulation of urban residential development indicates that 0.5-inch off of impervious surface infiltration standard reduces by 80% the average annual runoff volume over the case where no infiltration is used. Though this assumption would have to be verified and subsequently redefined for other land uses, it becomes a useful benchmark in determining what can be done to eliminate new and expanded discharges. As a starting point, this standard was applied to subwatershed ER04 to determine its adequacy in meeting the Baseline loads. Table 2 presents a comparison of loading rates with and.without the infiltration requirement. E!k Rrver 1481-0 700 1 Nondegradation Report Page 4